Gas turbine engines are required to perform at low emission levels and low noise levels during full power operation. Ideally any modifications made to a combustor to achieve lower emission levels or lower noise levels do not involve any compromise in durability or reliability.
At the compressor exit, testing indicates that pressure fluctuations include a mix of broadband low frequency signals and high frequency signals that are not solely attributable to acoustic causes. Attenuation of a broadband low and high frequency signals occurs in the combustion chamber and signals are dissipated in the turbine stage. At all engine speeds tone free low frequency signal are generated by the combustor. Pure acoustic propagation would show that combustor frequency ranges and far field would be related to the compressor pressure fluctuations by a simple time delay. This has not been found to be the case but rather the combustor itself is a source of far field low frequency noise.
It is an object of the present invention to provide a simple solution to enhance the acoustic transmission loss through the turbine stage and therefore to improve the overall engine noise level. Noise reduction techniques are of course well known however to date there appears to be no recognition that pressure fluctuations at the compressor exit are coupled with low frequency noise from the combustor.
For example, U.S. Patent Application Publication No. US2002/0073690 to Tse discloses an exhaust from a gas turbine engine with perforations to reduce noise level caused by exhaust mixing with bypass airflow from the turbine fan engine.
An object of the present invention however is to improve acoustic transmission loss through the turbine without compromising engine durability or reliability at minimum cost.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.